This invention relates to a digital mobile communication system, such as a Vehicular telephone system, a portable telephone system, a cordless telephone system, a radio LAN system, and more particularly to a system using a Code Division Multiple Accessing (CDMA) method as a radio accessing scheme between a base station and mobile stations.
In recent years, a spread spectrum communication system, which has good tolerance resistance to interference and jamming disturbance, has attracted attention as one of the communication scheme suitable for the mobile communication system.
In the radio communication system using the spread spectrum communication system, data is transmitted as follows, for example. The equipment on the transmission side first modulates the digitized audio data and video data by a digital modulation scheme, such as the PSK or FSK modulation scheme. Next, the modulated transmission data is converted into a wide-band base-band signal using spread code, such as pseudorandom noise code (PN code). Then, the spread transmission signal is converted into a radio-frequency signal, which is transmitted. The equipment device on the reception side performs an inverse spread operation using the same code as the spread code used in the transmission-side equipment. Then, the inverse spread reception signal is demodulated in a digital demodulation scheme, such as the PSK or FSK demodulation scheme to reproduce the data.
As a radio accessing scheme in this type of system, a Code Division Multiple Accessing (CDMA) method has been used which enables a plurality of mobile stations to communicate in the same frequency band simultaneously by allocating a different spread code to each mobile station.
In the CDMA mobile communication system, interference between channels limits the system capacity. To increase the system capacity, it is important to suppress interference between channels. Various interference suppressing techniques have been developed. One of them is to orthogonalize channels mutually by codes or spaces. In orthogonalizing channels by codes, use of orthogonal codes for which the cross-correlation value between codes is zero is effective. Orthogonal codes of this type include the Walsh code and the orthogonal Gold code.
With the orthogonal code, however, when the phase between channels is shifted, the cross-correlation value is not zero. Therefore, to suppress interference between channels using orthogonal codes, it is necessary to force the phases of orthogonal codes to synchronize each other. Generally, in a down-link channel transmitting signal from the base station to each mobile station, the transmission phase of each down-link channel signal is the same when the base station transmitted the signal. Since all of the down-link channel signals reach the corresponding mobile stations by way of the same transmission path, the cross-correlation value of the orthogonal codes between the individual down-link channel signals received at the respective mobile stations is zero as shown in FIG. 15. Therefore, it is possible to suppress interference between channels and achieve high-quality transmission.
In contrast, in an up-link channel transmitting signal from each mobile station to the base station, the distance between each mobile station and the base station, that is, the propagation delay time of an up-link channel signal transmitted from each mobile station to the base station, differs depending on the location of each mobile station. As a result, even if each mobile station transmits an up-link channel signal with the same phase, the reception timing of each up-link channel signal at the base station will differ from each other as shown in FIG. 16. Then, the cross-correlation value of the orthogonal codes between the up-link channel signals is not zero. Consequently, the interference between the up-link channels occurs, and, the system capacity doesn't increase.